1. Field of the Invention
The present invention relates to an image forming apparatus and a method for forming an image, and more particularly, to an image forming apparatus such as a digital color copy machine comprising means for setting a proper bias voltage of laser emitting means, and a method therefor.
2. Description of the Related Art
A conventional digital color copying machine comprises an image reading section for reading an image of a document (referred to as a document image hereinafter) using a color image sensor and converting the read image into binary print output signal, and a printer section for printing an image on a piece of copying paper in the electrophotographic process responsive to the binary print output signal. In the case of printing an image of plural colors on a piece of copying paper, the reading operation of the image reading section for reading a document image and the printing operation of the printer section for printing an image of each color on the same copying paper are performed sequentially every image of each color.
The above-mentioned printer section amplifies the print output signal inputted from the image reading section and output the amplified print output signal to a laser diode so as to drive it, and a beam of laser light emitted from the laser diode is scanned and projected onto a photoconductive drum so as to form an image thereon. In this case, in order to compensate a change in the emission amount of the laser diode caused due to a change in the temperature, a bias component having a threshold level is overlapped on the image signal. However, even though the image signal is in an off level, the laser diode slightly emits a beam of light (referred to as a bias light hereinafter) responsive to the above-mentioned bias component. As a result, a small amount of toner uniformly adheres onto a reproduced image responsive to the image signal, causing a so-called "fogging" phenomenon. In order to prevent the above-mentioned "fogging" phenomenon, it is disclosed in the U.S. Pat. No. 4,728,989 to provide means for lowering the intensity of a beam of laser light emitted from the laser diode in the transmission path of the laser light.
Also, in order to cut off unnecessary wavelength region component among a beam of laser light emitted from the laser diode in the printer section, it is disclosed in the U.S. Pat. No. 4,679,057 to provide in the transmission path of the laser light an optical filter for cutting off a wavelength region component having a wavelength lower than that of the laser light emitted when a threshold driving current supplied to
In order to drive the laser diode at a high speed in the printer section, it is necessary to previously supply a predetermined threshold current as a bias component to the laser diode prior to driving the laser diode. In this case, there is also such a problem that the above-mentioned "fogging" phenomenon is caused due to the bias light, as well as the case of the above-mentioned temperature compensation.
In order to overcome this problem, it is supposed that the light amount of the bias light incident to the photoconductive drum is lowered by using the above-mentioned means for lowering the light intensity and the above-mentioned optical filter. However, when the light output characteristic of the laser diode changes due to a change in environment such as a change in the temperature, there may be a probability of causing the "fogging" phenomenon again.